Typically, industrial cattle start their lives on grass. When they reach a certain age/weight, they are sold at auction and head to the feedlot where they get their corn/soy/fat/antibiotic slurry and gain weight at an incredible rate. The antibiotics are only there to head off problems inherent to feeding a ruminant a diet so unnatural. They're there for just a matter of a few months before slaughter (when they reach the appropriate weight). That's grain-finishing as I understand it.

The Omnivore's Dillemma by Michael Pollan is the book to read to understand food production in the industrial, industrial organic, and small-scale organic/"beyond organic" food production systems. I highly recommend it.